A new tetrahydroxanthene-1,3(2H)-dione derivative (1) was isolated from the leaves of Uvaria cordata collected in Viet Nam. Its structure was elucidated to be a mixture of four tautomers (1a–1d) by a combination of extensive spectroscopic analyses and the theoretical calculation of Gibbs free energies. Compound 1 exhibited moderate cytotoxicity against KB, LNCaP, Hep-G2, MKN-7, SW-480, HL-60, and SK-Mel-2 cancer cell lines with IC50 values ranging from 25.92 ± 2.33 to 44.29 ± 4.36 μg/mL. In addition, the previously reported structure of valderramenol A has been revised to 1a/1b.
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